This invention relates to a series regulator type power supply circuit.
FIG. 3A shows a circuit arrangement of a series regulator type power supply circuit conventionally used for an ECU (i.e., electronic control unit) installed in an automotive vehicle. As shown in FIG. 3A, a power supply circuit 1 includes a control IC (i.e., integrated circuit) 2 manufactured by the CMOS processes, a transistor Q1 for lowering or reducing the voltage, a transistor Q2 for activating the transistor Q1, a plurality of resistors R1-R4, and a reverse connection protecting diode D1. The power supply circuit 1 has a power input terminal 3 receiving a battery voltage VB supplied from a battery (not shown). The power supply circuit 1 has a terminal 4 producing a constant voltage of 5V under the constant voltage control performed by the IC 2.
The IC 2 includes a reference voltage circuit 5 (e.g., a band-gap reference voltage circuit) for generating a reference voltage Vr, an output voltage detecting circuit 6 consisting of two resistors R5 and R6 which are serially connected, an operational amplifier 8 for controlling the transistor Q2 via a terminal of IC2 based on a difference between the reference voltage Vr and a detection voltage Va, a clamp circuit 9 for supplying a power supply voltage (approximately 5V) to the reference voltage circuit 5 and to the operational amplifier 8, and other circuits operating in response to the generated constant voltage of 5V.
The clamp circuit 9, as shown in FIG. 3B, includes a plurality of P-channel transistors Q3 to Q6 and an N-channel transistor Q7. Each of the plurality of transistors Q3 to Q7 has a gate and a drain directly or commonly connected to each other. The battery voltage VB entering from the power input terminal 3 is applied to the clamp circuit 9 via the diode D1, the resistor R4, and a terminal 10 of IC2.
The resistor R4, determining a clamp current ICLMP supplied to the clamp circuit 9, has a relatively small resistance value so that the a sufficient amount of operation current can be supplied to each of the reference voltage circuit 5 and to the operational amplifier 8 even when the battery voltage VB is reduced to a minimum voltage level (e.g., 8V). The clamp current ICLMP increases with increasing battery voltage VB. The current consumption in the power supply circuit 1 increases correspondingly. Especially, when the power supply circuit 1 is used for the ECU or another automotive device mounted on a vehicle body, the power consumption of the battery increases.